A digital versatile disc (DVD) is known as an optical disc capable of recording a large volume of data, since it can record digital information about six times as much as a compact disc (CD). Recently, as an information amount to be recorded in an optical disc increases, an optical disc having a large capacity is required. In order to make an optical disc have a large capacity, it becomes necessary to increase the recording density of information by miniaturizing the optical spot to be defined by a ray of light irradiated onto the optical disc when the information is recorded on the optical disc and the information recorded on the optical disc is reproduced. For example, the optical spot can be miniaturized by shortening the wavelength of a laser beam from a light source, and increasing the numerical aperture (NA) of an objective lens. When conducting recording and reproducing of a DVD, a light source having a wavelength of 660 nm and an objective lens having a numerical aperture (NA) of 0.6 are used. Further, for example, by using a blue laser having a wavelength of 405 nm and an objective lens having a numerical aperture (NA) of 0.85, the recording density can be made to have five times as much as that of the current DVD.
An optical information apparatus capable of conducting high density recording and reproducing using a laser beam having a short wavelength by a blue laser is provided with a compatible function capable of conducting for an existing optical disc, which thus makes it possible to raise the usability of the optical information apparatus, and improve the cost performance. In this case, it is difficult to render the objective lens to have a high numerical aperture of 0.85 and a long operational distance (a space between the objective lens and an optical disc) as an objective lens for DVD or CD. For this reason, in a compatible-type optical information apparatus capable of conducting recording and reproducing at a high density, it is preferable to provide at least one objective lens used for conducting recording and reproducing onto and from CD or DVD, and another objective lens having a numerical aperture larger than the one objective lens and adapted for conducting high density recording.
Meanwhile, an objective lens actuator for an optical information apparatus is provided with a magnetic circuit for driving an objective lens both in a focusing direction and in a tracking direction. The magnetic circuit holds the space between the optical disc and the objective lens constant in the focusing direction, and moves the objective lens to a desired track position in the tracking direction. However, as mentioned above, in the optical information apparatus of performing compatibly for a plurality of optical discs having different recording densities, there is the necessity of providing objective lenses respectively corresponding to the plural optical discs having different recording densities. For this reason, the objective lens actuator is required to have a configuration where a plurality of objective lenses are mounted in a movable member, and are moved in the focusing direction and the tracking direction.
For example, Patent Document 1 discloses an optical head apparatus which is provided with a plurality of optical lenses on a movable member to thereby realize a compatible-type optical information apparatus which is capable of conducting recording and reproducing onto and from a plurality of optical discs having different recording densities (first prior art).
FIG. 17 shows a configuration of the optical head apparatus according to the first prior art. In this optical head apparatus, a light beam 61 emitted from a first light source (not shown) is regulated by a collimator lens 62 to have a parallelism, and deflected at a flat-shaped up-orienting mirror 63 so that an optical axis becomes perpendicular to an optical disc 65 having a high recording density. An objective lens 64 focuses the light beam 61 on a recording surface of the optical disc 65. A light beam 66 emitted from a second light source (not shown) is regulated by a collimator lens 67 to have a parallelism, and deflected at a flat-shaped up-orienting mirror 68 so that an axis becomes perpendicular to an optical disc 70 having a low recording density. An objective lens 69 focuses the light beam 66 on the recording surface of the optical disc 70.
An objective lens driving apparatus (objective lens actuator) 71 can move the first objective lens 64 both in the focusing direction F perpendicular to the recording surface of the optical disc 65 having a high recording density and in the tracking direction T of the optical disc 65. Further, an objective lens driving apparatus (objective lens actuator) 72 can move the second objective lens 69 both in the focusing direction F perpendicular to the recording surface of the optical disc 70 having a low recording density and the tracking direction T of the optical disc 70.
The objective lenses 64 and 69 are respectively fixedly attached at predetermined positions of the objective actuators 71 and 72.
Further, Patent Documents 2 and 3 disclose that two objective lenses are mounted on a single objective lens actuator, for example (second and third prior arts).    Patent Document 1: Japanese Unexamined Patent Publication No. 2002-208173,    Patent Document 2: Japanese Unexamined Patent Publication No. Hei. 11-120587, and    Patent Document 3: Japanese Unexamined Patent Publication No. 2002-245650.